Hmm, yes, I'm afraid the help is a little misleading here. Although the Invite Member page's inline help does say that you can bulk upload users, this feature is only available to sitewide administrators of the system (such as myself), and not to community site administrators. (There are good reasons for this, rather than it just being an arbitrary decision on our part!)
Hopefully you don't have so very many that inviting them individually is a big problem!
In the longer term we could look at enabling the buik inviting of users, though I don't know how important this is to the majority of sites.
Thanks
Joe - Voice Admin